The problem of ascertaining the identity of an individual or verifying whether an individual is the person he claims to be is a common one continually faced by individuals, businesses, and governments. Methods for positive identification of an individual usually include reliance upon knowledge of restricted information (such as a password), possession of a restricted article (such as a passkey), or physical appearance (such as matching a reference photo).
Security based upon knowledge or possession may be compromised without discovery since the information or article may be extorted from its rightful owner. The third methodology, commonly referred to as biometric techniques, is considered less vulnerable to mistaken identity. The best known biometric technique and the one considered most secure is fingerprint analysis. However, other biometric techniques exist. The present invention relates to an identification technique utilizing thermal imaging.
Skin temperature is a result of the balance between the heat flow from tissue to the skin surface and the heat flow from the surface to the surroundings. The basic mechanisms for transport of heat from the tissues to the skin surface include conduction throughout the tissues and convection by the flow of blood from the warm arterial supply through the capillary bed into the venous plexus. Metabolic heat generation is a second order influence compared to the convection transport mechanism of the blood perfusion. Thus in the absence of active superficial veins, the diffuse avascular skin temperature is directly related to the blood perfusion per unit volume of tissue, including the skin itself as well as tissues approximately 2 to 3 cm deep to the skin. Active superficial veins are also a possible indicator of regional blood flow. These are demonstrated as well-defined patterns of increased skin temperature. Skin temperature immediately overlying the vein is only a few tenths of a degree Celsius lower than the temperature of the blood flowing in the vein itself.
A variation of 3.5 degrees Fahrenheit is usual across a human facial surface. In the case of severe medical abnormalities, or when the ambient temperature is severe, the range may be twice as large, and its average value may vary. The heat energy given off by the face is changed to electrical energy and greatly amplified by the electronic circuitry of a thermal imaging system. Following amplification, the electrical energy is digitized. A facial "thermogram" is the resultant two-dimensional image obtained by scanning each section of the face onto one or more detectors which are sensitive in the infrared range. The most commonly used medical thermal imagers produce an image which comprises approximately 60,000 individual heat recordings. Most commonly used scanning infrared cameras do not accurately measure vein temperature because the modulation transfer function of the optics of these cameras produces a measurement that is related to the average temperature over almost 1 cm.sup.2 of scanning surface. Thus the size of the vein will influence the indicated temperature.
Because a thermograph operates at a distance from the subject and detects and records only radiant heat spontaneously emitted from the body surface, it constitutes a painless, non-invasive, passive method of recording body surface temperatures. In general, the nose, cheeks, ears, and the avascular corneas of the eyes are cool, as are the hair-insulated eyebrow, eyelash, and scalp regions. The anterior portion of the forehead is warm and has a narrow range of temperature variation over its surface. Heat is normally trapped by skin folds, at the canthi and along the free margins of the eyelids. Essentially symmetrical facial thermograms are found almost without exception in healthy subjects.
In accordance with the present invention, facial thermograms are processed to extract elemental shapes therefrom. The elemental shapes--like a fingerprint--are unique to an individual. Thus, the identity of an individual may be ascertained by comparing current elemental shapes with those from a reference.